[leetcode]Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes’ values.

For example:
Given binary tree {1,#,2,3},

1
\
2
/
3

树的前序遍历

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<int> ans;    vector<int> preorderTraversal(TreeNode *root) {        if(root == NULL) return ans;        ans.push_back(root->val);        preorderTraversal(root->left);        preorderTraversal(root->right);        return ans;    }};